OpenGL Introduction

By Nicholas Duchon.

These notes are based on the OpenGL Game template in Xcode 5.

Graphics frameworks

iOS Technology Overview: Media Layer

Framework	Link to reference	Comments.
UIKit	UIKit Framework Reference	Draw images, animate along paths
Core Graphics	Core Graphics Framework Reference Quartz 2D Programming Guide: Introduction	Path-based animation, drawing and image rendering
Core Animation	Core Animation Programming Guide: Core Animation Reference Collection	The animation part of Quartz 2D
Core Image	Core Image Reference Collection	Image processing and feature detection
OpenGL ES	OpenGL ES Programming Guide for iOS: OpenGL ES Framework Reference	3D graphics pipeline configuration
GL Kit	GLKit Framework Reference	Objective-C interface to OpenGL (partial)
Text Kit	Text Programming Guide for iOS:	Display, select and edit text
Core Text	Core Text Programming Guide: Core Text Reference Collection	Text layout and fonts - low level
Image I/O	Image I/O Reference Collection	Image I/O, like it says
Assets	Assets Library Framework Reference	Access pictures and video from Photos app
Spite Kit	Sprite Kit Programming Guide: code:Explained Adventure: Introduction	Basic elements of creating a animated game Cool example.

When you run the template project, you see the following:

The original vertex data array:

I will experiment with object definition for a while, starting with the following code - the cube specification from viewController.m file, lines 30-75.
(You can turn on line numbers in Xcode by going to Xcode --> Preferences --> Text Editing --> Show: Line Numbers.)

You can use the following code to compute the number of vertices and the size of the array, AFTER you have declared the array without a size specification:

int sB = sizeof (gCubeVertexData); // size of array in bytes int se = sizeof(gCubeVertexData[0]);// size of an entry in bytes, float is typically 4 bytes int pointsInCube = sB / se / 6; // divide by 6 since 6 values per vertex, xyz location, xyz normals

GLfloat gCubeVertexData[] = // ND: I removed the 216 here - the compiler will infer the size, which is better than declaring it{ // Data layout for each line below is: // positionX, positionY, positionZ, normalX, normalY, normalZ, .5f, -0.5f, -0.5f, 1.0f, 0.0f, 0.0f, .5f, 0.5f, -0.5f, 1.0f, 0.0f, 0.0f, .5f, -0.5f, 0.5f, 1.0f, 0.0f, 0.0f, .5f, -0.5f, 0.5f, 1.0f, 0.0f, 0.0f, .5f, 0.5f, -0.5f, 1.0f, 0.0f, 0.0f, .5f, 0.5f, 0.5f, 1.0f, 0.0f, 0.0f, 0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f, -0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f, 0.5f, 0.5f, 0.5f, 0.0f, 1.0f, 0.0f, 0.5f, 0.5f, 0.5f, 0.0f, 1.0f, 0.0f, -0.5f, 0.5f, -0.5f, 0.0f, 1.0f, 0.0f, -0.5f, 0.5f, 0.5f, 0.0f, 1.0f, 0.0f, -0.5f, 0.5f, -0.5f, -1.0f, 0.0f, 0.0f, -0.5f, -0.5f, -0.5f, -1.0f, 0.0f, 0.0f, -0.5f, 0.5f, 0.5f, -1.0f, 0.0f, 0.0f, -0.5f, 0.5f, 0.5f, -1.0f, 0.0f, 0.0f, -0.5f, -0.5f, -0.5f, -1.0f, 0.0f, 0.0f, -0.5f, -0.5f, 0.5f, -1.0f, 0.0f, 0.0f, -0.5f, -0.5f, -0.5f, 0.0f, -1.0f, 0.0f, 0.5f, -0.5f, -0.5f, 0.0f, -1.0f, 0.0f, -0.5f, -0.5f, 0.5f, 0.0f, -1.0f, 0.0f, -0.5f, -0.5f, 0.5f, 0.0f, -1.0f, 0.0f, 0.5f, -0.5f, -0.5f, 0.0f, -1.0f, 0.0f, 0.5f, -0.5f, 0.5f, 0.0f, -1.0f, 0.0f, 0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f, -0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f, 0.5f, -0.5f, 0.5f, 0.0f, 0.0f, 1.0f, 0.5f, -0.5f, 0.5f, 0.0f, 0.0f, 1.0f, -0.5f, 0.5f, 0.5f, 0.0f, 0.0f, 1.0f, -0.5f, -0.5f, 0.5f, 0.0f, 0.0f, 1.0f, 0.5f, -0.5f, -0.5f, 0.0f, 0.0f, -1.0f, -0.5f, -0.5f, -0.5f, 0.0f, 0.0f, -1.0f, 0.5f, 0.5f, -0.5f, 0.0f, 0.0f, -1.0f, 0.5f, 0.5f, -0.5f, 0.0f, 0.0f, -1.0f, -0.5f, -0.5f, -0.5f, 0.0f, 0.0f, -1.0f, -0.5f, 0.5f, -0.5f, 0.0f, 0.0f, -1.0f};

The first question that comes to mind is "Where is GLfloat defined, and what is it defined as?"
Answer: Frameworks -->. OpenGLES.framework --> (ES1 or ES2 or ES3) --> gl.h --> line 55/65/57 in all three cases as the primitive data type "float"
SO: this is simply an array of float numbers

Structure and meaning:

Every triangle uses 3 points
I have added some more returns to show the triangles
each of the triangles uses a single normal (up direction) to all three points
You can experiment with these directions to see what happens as you change them.

Experimenting

Just one triangle

GLfloat gCubeVertexData[] = { // Data layout for each line below is: // positionX, positionY, positionZ, normalX, normalY, normalZ,// x y x nx ny nz 1, 1, 1, 1, 0, 0, 1, 1, -1, 1, 0, 0, 1, -1, 1, 1, 0, 0};

To change the magnification, change the location of the viewer as follows:
In the "update" method,
Change 65 to some other number. The number I used to make the video is 125. Bigger numbers put the camera viewpoint further away, making the figures smaller, while smaller numbers bring the camer closer.
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(125), aspect, 0.1f, 100.0f);

For some fun, what does the following set of points specify?

GLfloat gCubeVertexData[] ={ // Data layout for each line below is: // positionX, positionY, positionZ, normalX, normalY, normalZ,// x y x nx ny nz 1, -1, -1, 1, 0, 0, 1, 1, -1, 1, 0, 0, 1, -1, 1, 1, 0, 0, 1, -1, 1, 1, 0, 0, 1, 1, -1, 1, 0, 0, 1, 1, 1, 1, 0, 0,};

Animation

The system can be configured using the property preferredFramesPerSecond in the GLKViewController class. Default is 30, but the actual frame rate depends on the capabilties of the device.

If your class extends GLKViewController, as does the class ViewController in the template, you can just say:
self.preferredFramesPerSecond = 20; // or something similar.
Once that is set, the system will generate events at that rate. The next question is how to handle the events.

The documentation is not easy to find, and I am still not clear how the compiler actually finds the key method, but the recommended method name in a class that extends GLKViewController is -(void) update.
You can see an example of this method in the ViewController.m file.

For documentation of the update method, you can look in the GLKViewController.h file (in the GLKit.Framework folder of an Xcode project), lines 86-90 as comments:
/* Required method for implementing GLKViewControllerDelegate. This update method variant should be used when not subclassing GLKViewController. This method will not be called if the GLKViewController object has been subclassed and implements -(void)update. */- (void)glkViewControllerUpdate:(GLKViewController *)controller;

You can also find a reference to the update method in the reference page for the class GLKViewController - Subclassing Notes.